Sealed Batteries vs. Unsealed Batteries

Newly devised automobile batteries are sealed. When lead storage batteries discharge, they produce hydrogen. Do you know why sealed lead storage batteries have not been used in the past?

Is it because the hydrogen gas is hazardous under such high pressure and back then, they couldn't sustain a safe temperature for the batteries since no such technology was available yet or well-equipped for that job?

I had to estimate the increase in pressure if 0.05 grams of hydrogen gas was produced and it was around 6 atm.